The geographic range of Carex prairea is rather fragmented except for a core zone that extends from southern Quebec and New England to Alberta. Oregon, Idaho, Wyoming, and California reports should probably be referred to C. cusickii or some other species, because south of the Canadian border (except for one locality in western Montana), C. prairea is unknown from west of the Great Plains. Carex appropinquata Schumacher, reported from Manitoba by W. J. Hooker ([1829–]1840, as C. paradoxa Willdenow) and from Colorado by L. Kelso (1953, 1953b), must be excluded. Hooker’s citation possibly refers to C. praegracilis W. Boott or less likely to C. prairea, and Kelso’s specimens (in US) are C. praegracilis.

Carex prairea and C. diandra are closely related and may not appear to be sharply differentiated. However, they maintain separate identities throughout their overlapping ranges and prefer subtly different habitats. Carex prairea is strictly calcicolous, avoiding saline groundwater and oligotrophic muskegs, whereas C. diandra does not require calcareous conditions and so is more likely to be found, for example, in New England and the Canadian Shield region.